With demands for a reduction in power consumption, the voltage of integrated circuits such as a CPU (Central Processing Unit) is reduced more and more. On the other hand, in some cases, a relatively high voltage is needed because of higher functionalities or compatibility with conventional systems. For example, in a power supply for driving a CPU, it is necessary to switch supply voltages because the CPU changes power consumption by switching states. In a power supply for driving an IC card mounted with a memory or the like, it is necessary to switch and supply a supply voltage in order to meet a plurality of specifications having different operating voltages. With an increase in the speed of devices, high speed switching is demanded for switching supply voltages. However, in the case of decreasing a supply voltage, the supply voltage does not reach a desired voltage until electric charges stored in a capacitance between an output terminal and a ground terminal are discharged, so an increase in the speed is restricted.